The present invention relates to magnetic resonance imaging systems, apparatus and procedures. In magnetic resonance imaging, an object to be imaged as, for example, a body of a human subject, is exposed to a strong, substantially constant static magnetic field. The static magnetic field causes the spin vectors of certain atomic nuclei within the body to randomly rotate or “precess” around an axis parallel to the direction of the static magnetic field. Radio frequency excitation energy is applied to the body, and this energy causes the precessing automatic nuclei to rotate or “precess” in phase and in an excited state. As the precessing atomic nuclei relax, weak radio frequency signals are emitted; such radio frequency signals are referred to herein as magnetic resonance signals.
Different tissues produce different signal characteristics. Furthermore, relaxation times are the dominant factor in determining signal strength. In addition, tissues having a high density of certain nuclei will produce stronger signals than tissues with a low density of such nuclei. Relatively small gradients in the magnetic field are superimposed on the static magnetic field at various times during the process so that magnetic resonance signals from different portions of the patient's body differ in phase and/or frequency. If the process is repeated numerous times using different combinations of gradients, the signals from the various repetitions together provide enough information to form a map of signal characteristics versus location within the body. Such a map can be reconstructed by conventional techniques well known in the magnetic resonance imaging art, and can be displayed as a pictorial image of the tissues as known in the art.
The magnetic resonance imaging technique offers numerous advantages over other imaging techniques. MRI does not expose either the patient or medical personnel to X-rays and offers important safety advantages. Also, magnetic resonance imaging can obtain images of soft tissues and other features within the body which are not readily visualized using other imaging techniques. Accordingly, magnetic resonance imaging has been widely adopted in the medical and allied arts.
Several factors affect the positioning of patients and ancillary equipment in MRI imaging. Many MRI magnets use one or more solenoidal superconducting coils to provide the static magnetic field arranged so that the patient is disposed within a small tube running through the center of the magnet. The magnet and tube typically extend along a horizontal axis, so that the long axis or head-to-toe axis of the patient's body must be in a horizontal position during the procedure. Moreover, equipment of this type provides a claustrophobic environment for the patient.
Iron core magnets have been built to provide a more open environment for the patient. These magnets typically have a ferromagnetic frame with a pair of ferromagnetic poles disposed one over the other along a vertical pole axis with a gap between them for receiving the patient. The frame includes ferromagnetic flux return members such as plates or columns extending vertically outside of the patient-receiving gap. A magnetic field is provided by permanent magnets or electromagnetic coils associated with the frame. A magnet of this type can be designed to provide a more open environment for the patient. However, it is still generally required for the patient to lie with his or her long axis horizontal. This affects the conditions under which imaging may be performed and also affects the comfort level of the patient.
Recently, magnets having horizontal pole axes that provide an open patient environment have been developed. As disclosed, for example, in commonly assigned U.S. Pat. No. 6,414,490 (the “'490 patent”), which is a continuation of U.S. patent application Ser. No. 08/978,084, filed on Nov. 25, 1997, and in co-pending, commonly assigned U.S. patent application Ser. No. 09/718,946 (the “'946 application”), filed on Nov. 22, 2000, the disclosures of which are incorporated by reference herein, a magnet having poles spaced apart from one another along a horizontal axis provides a horizontally oriented magnetic field within a patient-receiving gap between the poles. Such a magnet can be used with a patient positioning device including elevation and tilt mechanisms to provide extraordinary versatility in patient positioning. For example, where the patient positioning device includes a bed or similar device for supporting the patient in a recumbent position, the bed can be tilted and/or elevated so as to image the patient in essentially any position between a fully standing position and a fully recumbent position, and can be elevated so that essentially any portion of the patient's anatomy is disposed within the gap in an optimum position for imaging. As further disclosed in the aforesaid applications, the patient positioning device may include additional elements such as a platform projecting from the bed to support the patient when the bed is tilted towards a standing orientation. Still other patient supporting devices can be used in place of a bed in a system of this type. For example, a seat may be used to support a patient in a sitting position. Thus, magnets of this type provide extraordinary versatility in imaging.
The position of a patient during magnetic resonance imaging may affect or limit the imaging information obtained. A patient may exhibit a symptom if oriented in an upright or weight bearing position and no symptom if oriented in a recumbent or horizontal position. For example, it may be necessary to image a patient in an upright or gravity bearing position to discern a symptom and provide a diagnosis for injuries relating to the neck, spine, hip, knee, foot or ankle areas of the human anatomy.
In addition to a patient's position, movement by a patient during imaging may also affect the images obtained. In fact, magnetic resonance imaging procedures generally require the patient to remain perfectly still during imaging. A patient positioned in a weight-bearing upright posture may find it more difficult to remain still during imaging. The anxiety level of a patient is another factor that may affect how still a patient remains during imaging. In general, those magnets that place the patient in the bore of the magnet during imaging tend to add to the patient's anxiety level because of the closed-in and tight environs. A more relaxed patient tends to move less during imaging.
It is often desirable to provide fixtures in close proximity to the patient. For example, local antennas such as small solenoidal coils can be placed around a part of the patient's body to be imaged as, for example, around the head or around a limb of the patient. These antennas can be used to transmit the RF excitation signals, to receive the magnetic resonance signals emitted by the tissue, or both. Such local antennas allow improved reception of signals from the specific region of interest within the patient's body. Other fixtures can be used for purposes such as supporting or positioning parts of the patient's body relative to the table as, for example, a rest for supporting the patient's head or limb. Typically, these fixtures are simply placed on the surface of the bed at the desired location for a particular patient, or are placed on the patient's body so that the fixture will be supported by the bed surface when the patient lies on the bed surface. These arrangements are satisfactory where the bed remains in a horizontal position at all times.
Where the patient support table is in a generally vertical orientation during all or a portion of the procedure, fixtures positioned on the surface of the support will fall off of the support unless they are secured to the surface. Although the fixtures can be secured to the support using devices improvised for a particular application, as, for example, straps or tape, such arrangements do not offer a complete solution. Accordingly, there has been a need for improved apparatus for positioning fixtures in magnetic resonance apparatus, and for magnetic resonance apparatus incorporating such improved positioning apparatus. In particular, there is a need for fixtures that can immobilize a patient's head and neck in a variety of orientations without adding to a patient's anxiety.